What does this MR do (summary)?
What changes have you made (detailed)?
Allow the list of rooms for a session to be empty, in case a session is held online.
Does this MR meet the acceptance criteria?
The main application was run to make sure the server still runs without errors.
Tests were written to verify the behaviour of this code.
Javadoc and other comments were added to make the code understandable.
The code adheres to the style guidelines.